Nanoimprint lithography is a method of fabricating nanometer scale patterns by mechanical deformation of imprint resist and subsequent processing. The imprint resist can be a thermally softened or photo-initiated liquid coating that is cured by heat or UV light during the imprinting. A template is brought into contact with the liquid coating and the liquid is cured. The cured liquid includes an imprint of any patterns formed in the template. Alignment of the template with the substrate is performed prior to curing the liquid as described in U.S. Pat. No. 6,916,584. Adhesion between the resist and the template must be controlled to allow proper release, see U.S. Pat. No. 7,157,036. The subject matter of both patents is incorporated herein.
A nano-pattern “parent” is produced using lithography on a silicon or glass parent. The parent pattern, sometimes called a positive image, is created using durable or environmentally stable materials, for example, a chrome positive created on glass. The pattern is then replicated on a liquid Polydimethylsiloxane (PDMS) layer, sometimes called a child layer or negative image. The PDMS is then cured and the final image is used as a template or stamp to reproduce the image on multiple products. The PDMS child pattern is then replicated onto another liquid layer, for example an epoxy-based negative photoresist (SU-8), re-creating the original positive image. SU-8 can be hardened using a combination of light and heat.
Despite the good properties of PDMS, there is a possibility of mechanical stress and thermal expansion causing errors in the moldable layer. As a result, U.S. Pat. No. 7,704,425 teaches performing all processing steps when using the stamp to transfer a pattern to a substrate at a constant control temperature, which is inconvenient in a manufacturing environment.
Wilhelm (Thesis, Massachusetts Institute of Technology, June 2001) teaches casting the stamp around spring steel. As a result of the stresses, however, and under repeated bends, separation of the elastomer from the substrate or steel will occur resulting in waste and short life for the template. Wilhelm also identifies a significant problem with stamping using a flat fixed stamp with air bubbles. Air bubbles trapped between the stamp and the liquid substrate which receives the pattern transfer, prevents good contact between the stamp and substrate thereby resulting in pattern transfer defect. Wilhelm suggests a stiff bowed stamp as a solution. The bow in the stamp shape can help to push the air bubbles formed out from the center of the stamp. There is, however, a problem, there is too much contact force in the center of the stamp, where the stamp is at maximum height and poor pattern transfer at the edges of the stamp where the stamp is a minimum height. Wilhelm suggests the use of a thin flexible stamp to avoid the pattern transfer issues, but that results in wrinkles and poor pattern transfer.
What is needed is a reinforced elastomeric template or stamp. It must be resistant to mechanical stress and thermal expansion and have excellent durability. For UV curing during the pattern transfer process, the stamp must pass light even with the reinforcing substrate in place. Finally, the stamp must be able to take some curvature during its use to avoid the formation of bubbles and have a well controlled contact profile to avoid loss of quality due to failure to transfer the pattern in areas of excessive or insufficient contact.